i had to upgrade zimbra. there was no admin for 3 years. documentation is sparse.i upgraded from 8.0 at ubuntu 12.04 to 8.6, upgraded ubuntu to 14.04, installed 8.6 for 14.04, upgraded to 8.8.5 on 14.04, upgraded ubuntu to 16.04 and installed 8.8.5 for ubuntu 16.04

now the external mailserver cannot send mails to the internal zimbra anymore. i can send internal mails, i can send mails out from zimbra, but there are no mails coming in. i have a lot of messages like this on the external server